BaseTools/GenFw: allow AArch64 tiny and small code model relocations

The AArch64 small C model makes extensive use of ADRP/ADD and
ADRP/{LDR,STR} pairs to emit PC-relative symbol references with
a +/- 4 GB range. Since the relocation pair splits the relative
offset into a relative page offset and an absolute offset into
a 4 KB page, we need to take extra care to ensure that the target
of the relocation preserves its alignment relative to a 4 KB
alignment boundary.

Also, due to a problem with the --emit-relocs GNU ld option, where
it does not recalculate the addends for section relative relocations,
the only way to guarantee correct code is by requiring the relative
section offset to be equal in the ELF and PE/COFF versions of the
binary. This affects both the 'tiny' and 'small' GCC code models.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Tested-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18197 6f19259b-4bc3-4df7-8a09-765794883524

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