Work towards RISC-V cross-compilation support

[fwsGonzo]

This is probably not going to be a mergable PR, but I imagine when RISC-V support is added, this work will help. I'm using a RISC-V emulator to run build-generators instead of complicating the CMake script

[fwsGonzo]

After some testing it seems that everything works, so I'm undrafting this.

For building this on actual 64-bit RISC-V there are unnecessary changes:

• No RISC-V emulator is needed to run build-gen programs, like mksnapshot
• No need for -static or for the occasional -latomic work-around
• Including v8config.h in v8/src/heap/base/asm/riscv/push_registers_asm.cc is perhaps still necessary, but perhaps better done from CMake?

[bnoordhuis]

This is an accidental whitespace change, isn't it? If so:

Suggested change

	endif ()
	endif ()

[bnoordhuis]

s/tab/whitespace/

Suggested change

	$<${is-riscv}:v8/src/heap/base/asm/riscv/push_registers_asm.cc>
	$<${is-riscv}:v8/src/heap/base/asm/riscv/push_registers_asm.cc>

[bnoordhuis]

Suggested change

	set(mksnp-prefix ${RISCV_EMULATOR} -P --)
	set(mksnp-prefix ${RISCV_EMULATOR} -P --)

And consider naming it mksnapshot-prefix for clarity.

[bnoordhuis]

Suggested change

	set(torque-prefix ${RISCV_EMULATOR} -P --) # Proxy through RISC-V emulator
	set(torque-prefix ${RISCV_EMULATOR} -P --) # Proxy through RISC-V emulator

[bnoordhuis]

Suggested change

	-static
	-static

I half and half expect this to break Windows builds though.

[bnoordhuis]

Suggested change

	set(bblg-prefix ${RISCV_EMULATOR} -P --) # Proxy through the emulator
	set(bblg-prefix ${RISCV_EMULATOR} -P --) # Proxy through the emulator

And please name it less abbrev.

[bnoordhuis]

Patches to V8 should go into patches/, not be directly applied, otherwise the next update will blow them away.

But! - and this is important - changes should be submitted to upstream first, because I don't want to be forward-porting patches forever if I can avoid it. See https://v8.dev/docs/contribute for more info.