[TEST] Add support for link time optimization

- Code generation is not done until after linking, so the
compiler must be invoked at all linking stages instead of the linker. As
a consequence all linker arguments must be prefixed with -Wl.
- Partial linking is not supported. Instead, object files are collected
into thin archives that are linked instead.
- Wrapping libgcc functions causes a symbol mismatch when using LTO.
Wrapping these functions was originally done to support alternate
regparam values, though AFAICT this isn't used anywhere.

Using LTO leads to a ~10% decrease in stage size for QEMU and ~18% for
the Thinkpad T500, and both targets boot successfully.

Change-Id: I48c31ea8b1b57276125cffdac44c7c16642547ac
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
---
M Makefile.inc
M src/Kconfig
M src/arch/arm/Makefile.inc
M src/arch/arm64/Makefile.inc
M src/arch/ppc64/Makefile.inc
M src/arch/riscv/Makefile.inc
M src/arch/x86/Makefile.inc
M src/cpu/x86/smm/Makefile.inc
M src/lib/Makefile.inc
M src/lib/gcc.c
M toolchain.inc
M util/xcompile/xcompile
12 files changed, 61 insertions(+), 44 deletions(-)