@Asperamanca, I grep'd through mkspecs, and this is what I have in one of my qt 5.5 installations' mkspec folder:
$ grep -E -o -r 'QMAKE_PLATFORM.*?$'
aix-g++/qmake.conf: QMAKE_PLATFORM = aix
aix-g++-64/qmake.conf: QMAKE_PLATFORM = aix
aix-xlc/qmake.conf: QMAKE_PLATFORM = aix
aix-xlc-64/qmake.conf: QMAKE_PLATFORM = aix
android-g++/qmake.conf: QMAKE_PLATFORM = android
blackberry-armle-v7-qcc/qmake.conf: QMAKE_PLATFORM += blackberry
blackberry-x86-qcc/qmake.conf: QMAKE_PLATFORM += blackberry
common/ios.conf: QMAKE_PLATFORM += ios
common/linux-android.conf: QMAKE_PLATFORM += android
common/linux.conf: QMAKE_PLATFORM += linux
common/mac.conf: QMAKE_PLATFORM += mac darwin
common/macx.conf: QMAKE_PLATFORM += osx macx
common/msvc-desktop.conf: QMAKE_PLATFORM = win32
common/nacl/nacl-base.conf: QMAKE_PLATFORM = nacl $$QMAKE_PLATFORM
common/qcc-base-qnx.conf: QMAKE_PLATFORM += qnx
common/unix.conf: QMAKE_PLATFORM += unix posix
common/wince/qmake.conf: QMAKE_PLATFORM += wince win32
common/winrt_winphone/qmake.conf: QMAKE_PLATFORM = winrt win32
cygwin-g++/qmake.conf: QMAKE_PLATFORM = cygwin unix posix
darwin-g++/qmake.conf: QMAKE_PLATFORM = osx macx mac darwin
features/spec_post.prf: QMAKE_PLATFORM) {
features/spec_post.prf: QMAKE_PLATFORM = unix
features/spec_post.prf: QMAKE_PLATFORM = osx macx mac darwin unix
features/spec_post.prf: QMAKE_PLATFORM = win32
features/spec_post.prf: QMAKE_PLATFORM, macx):!contains(QMAKE_PLATFORM, osx) {
features/spec_post.prf: QMAKE_PLATFORM = osx $$QMAKE_PLATFORM
features/spec_post.prf: QMAKE_PLATFORM
freebsd-g++/qmake.conf: QMAKE_PLATFORM = freebsd bsd
freebsd-g++46/qmake.conf: QMAKE_PLATFORM = freebsd bsd
freebsd-icc/qmake.conf: QMAKE_PLATFORM = freebsd bsd
haiku-g++/qmake.conf: QMAKE_PLATFORM = haiku
hpux-acc/qmake.conf: QMAKE_PLATFORM = hpux
hpux-acc-64/qmake.conf: QMAKE_PLATFORM = hpux
hpux-acc-o64/qmake.conf: QMAKE_PLATFORM = hpux
hpux-g++/qmake.conf: QMAKE_PLATFORM = hpux
hpux-g++-64/qmake.conf: QMAKE_PLATFORM = hpux
hpuxi-acc-32/qmake.conf: QMAKE_PLATFORM = hpux
hpuxi-acc-64/qmake.conf: QMAKE_PLATFORM = hpux
hpuxi-g++-64/qmake.conf: QMAKE_PLATFORM = hpux
hurd-g++/qmake.conf: QMAKE_PLATFORM += hurd
irix-cc/qmake.conf: QMAKE_PLATFORM = irix
irix-cc-64/qmake.conf: QMAKE_PLATFORM = irix
irix-g++/qmake.conf: QMAKE_PLATFORM = irix
irix-g++-64/qmake.conf: QMAKE_PLATFORM = irix
linux-cxx/qmake.conf: QMAKE_PLATFORM = linux
linux-kcc/qmake.conf: QMAKE_PLATFORM = linux
linux-pgcc/qmake.conf: QMAKE_PLATFORM = linux
lynxos-g++/qmake.conf: QMAKE_PLATFORM = lynxos
netbsd-g++/qmake.conf: QMAKE_PLATFORM = netbsd bsd
openbsd-g++/qmake.conf: QMAKE_PLATFORM = openbsd bsd
sco-cc/qmake.conf: QMAKE_PLATFORM = sco
sco-g++/qmake.conf: QMAKE_PLATFORM = sco
solaris-cc/qmake.conf: QMAKE_PLATFORM = solaris
solaris-cc-64/qmake.conf: QMAKE_PLATFORM = solaris
solaris-g++/qmake.conf: QMAKE_PLATFORM = solaris
solaris-g++-64/qmake.conf: QMAKE_PLATFORM = solaris
tru64-cxx/qmake.conf: QMAKE_PLATFORM = tru64
tru64-g++/qmake.conf: QMAKE_PLATFORM = tru64
unixware-cc/qmake.conf: QMAKE_PLATFORM = unixware
unixware-g++/qmake.conf: QMAKE_PLATFORM = unixware
unsupported/freebsd-clang/qmake.conf: QMAKE_PLATFORM = freebsd bsd
unsupported/linux-host-g++/qmake.conf: QMAKE_PLATFORM = linux
unsupported/qnx-X11-g++/qmake.conf: QMAKE_PLATFORM = qnx
unsupported/vxworks-ppc-dcc/qmake.conf: QMAKE_PLATFORM = vxworks
unsupported/vxworks-ppc-g++/qmake.conf: QMAKE_PLATFORM = vxworks
unsupported/vxworks-simpentium-dcc/qmake.conf: QMAKE_PLATFORM = vxworks
unsupported/vxworks-simpentium-g++/qmake.conf: QMAKE_PLATFORM = vxworks
win32-g++/qmake.conf: QMAKE_PLATFORM = win32 mingw
win32-icc/qmake.conf: QMAKE_PLATFORM = win32
winphone-arm-msvc2013/qmake.conf: QMAKE_PLATFORM = winphone $$QMAKE_PLATFORM
winphone-x86-msvc2013/qmake.conf: QMAKE_PLATFORM = winphone $$QMAKE_PLATFORM