package/flashbench: remove old toolchain microblaze & sh2a workaround

package/flashbench: remove old toolchain microblaze & sh2a workaround

The old binary Microblaze and Sh2a toolchains were affected by Glibc
Bug 1228: https://sourceware.org/bugzilla/show_bug.cgi?id=1228 that
has been fixed in 2005, so let's remove actual work around:
'FLASHBENCH_CFLAGS += -std=gnu99' for both BR2_microblaze and BR2_sh2a
and remove local FLASHBENCH_CFLAGS in favour of TARGET_CFLAGS.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>