[flang] disable fast_math test on powerpc

There seems not be any crtfastmath.o on powerpc.

See https://reviews.llvm.org/D138675

Author: tblah

flang/test/Driver/fast_math.f90

@@ -57,7 +57,8 @@
 ! CHECK-CONTRACT-SAME: -ffp-contract=off
 
 ! Check that -ffast-math causes us to link to crtfastmath.o
-! UNSUPPORTED: system-windows, powerpc
+! UNSUPPORTED: system-windows
+! UNSUPPORTED: target={{p(ower)?pc.*}}
 ! RUN: %flang -ffast-math -### %s -o %t 2>&1 \
 ! RUN:     | FileCheck --check-prefix=CHECK-CRT %s
 ! CHECK-CRT: {{crtbegin.?\.o}}