mold is usually built for all supported tagets, namely, x86-64, i386,
ARM32, ARM64 and RISCV64. This is a good thing because it makes cross
compilation easy. That is, as long as you have a copy of the mold linker,
it is guaranteed to work as a cross linker.
However, during a quick debug session in which you build mold many
times, you may want to build mold only for your native target. That
greatly reduces build time because it reduces the amount of code
after template instantiation.
Therefore, in this commit, I introduced new macros,
MOLD_DEBUG_X86_64_ONLY and MOLD_DEBUG_ARM64_ONLY, to build mold for
x86-64 and ARM64 only, respectively.
These flags should never be used for production. They are solely for
debugging purpose.
As we add more targets to mold/ELF, it takes more time to compile
source files because compiler has to instantiate more templates.
I think we need to do something to fix it, but for now, I'll just
add a stub for RISC-V 64-bit ISA so that we can start working on
RV64.
We try to overwrite to an existing output file if exists because
writing to a file that is already in buffer cache is much faster than
creating a new fresh file and write to it.
Previously, mold failed with the "fchmod failed" error if an existing
file is writable (so open for read-write succeeds) but owned by
someone else (so fchown fails). Now, mold ignore such existing file
and create a fresh output file.
Fixes https://github.com/rui314/mold/issues/275
