arch: introduce BR2_GCC_TARGET_{FPU, FLOAT_ABI}

Buildroot already has the BR2_GCC_TARGET_{TUNE,ARCH,ABI,CPU} hidden
kconfig strings that allow per-architecture Config.in files to feed
the appropriate values of --with-{tune,arch,abi-cpu} when building
gcc, or the appropriate flags for the external toolchain wrapper.

This commit has two additional options:
BR2_GCC_TARGET_{FPU,FLOAT_ABI}, that allows to define the
--with-{fpu,float} gcc configure options for the internal backend, or
the -m{fpu,float-abi} options for the flags of the external toolchain
wrapper.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>