[powerpc] fegetenv_status: simplify instruction generation

fegetenv_status() wants to use the lighter weight instruction 'mffsl'
for reading the Floating-Point Status and Control Register (FPSCR).
It currently will use it directly if compiled '-mcpu=power9', and will
perform a runtime check (cpu_supports("arch_3_00")) otherwise.

Nicely, it turns out that the 'mffsl' instruction will decode to
'mffs' on architectures older than "arch_3_00" because the additional
bits set for 'mffsl' are "don't care" for 'mffs'.  'mffs' is a superset
of 'mffsl'.

So, just generate 'mffsl'.