toolchain/toolchain-wrapper: let recent GCC handle SOURCE_DATE_EPOCH

toolchain/toolchain-wrapper: let recent GCC handle SOURCE_DATE_EPOCH

When using precompiled headers, changing any macros defined on the
command line will invalidate the precompiled header.  With
toolchain-wrapper adding __DATE__ and __TIME__, any commits to Buildroot
will invalidate incremental builds regardless of whether the precompiled
header actually uses those values (affecting _OVERRIDE_SRCDIR).

GCC-7 and later support SOURCE_DATE_EPOCH and use it to define __DATE__
and __TIME__ internally, avoiding any impact on precompiled headers.

Disable the custom handling in toolchain-wrapper if GCC is version 7 or
newer.

Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>