The C code that is calling crosscall1 may depend on the GP register, which Go code
will currently clobber. Save and restore both X3 (aka GP) and X4 (aka TP) in this
code path (note that the Go code does not currently clobber X4, however there is
minimal downside to saving and restoring it here, which then also matches crosscall2).

Updates golang#47100

Change-Id: Icbb706d7889d5dc59de3efb2b510fa6ea2069496

The X3 (aka GP) register will potentially be loaded with the __global_pointer$ symbol
during program start up (usually the dynamic linker). As such, non-Go code may depend
on the contents of GP, including code called via cgo. Save the original GP value on
early entry to Go, then restore it in asmcgocall prior to calling non-Go code.

An alternative would be to prevent Go from allocating the X3 register, but this seems
like a better option.

Updates golang#47100

Change-Id: I10356886babd958da019a6a0179f609fe90517da

This provides the runtime glue (_rt0_riscv64_linux_lib) for c-archive and c-shared
support, along with enabling both of these buildmodes on linux/riscv64.

Both misc/cgo/testcarchive and misc/cgo/testcshared now pass on this platform.

Fixes golang#47100

Change-Id: I7ad75b23ae1d592dbac60d15bba557668287711f

The C code that is calling crosscall1 may depend on the GP register, which Go code
will currently clobber. Save and restore both X3 (aka GP) and X4 (aka TP) in this
code path (note that the Go code does not currently clobber X4, however there is
minimal downside to saving and restoring it here, which then also matches crosscall2).

Updates #47100

Change-Id: Icbb706d7889d5dc59de3efb2b510fa6ea2069496
Reviewed-on: https://go-review.googlesource.com/c/go/+/334870
Trust: Joel Sing <joel@sing.id.au>
Trust: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Meng Zhuo <mzh@golangcn.org>

The X3 (aka GP) register will potentially be loaded with the __global_pointer$ symbol
during program start up (usually by the dynamic linker). As such, non-Go code may depend
on the contents of GP and calculate offsets based on it, including code called via cgo
and signal handlers installed by non-Go code. As such, stop using the X3 register so
that there are fewer issues interacting between Go and non-Go code.

While here remove the X4 (TP) name from the assembler such that any references must
use the 'TP' name. This should reduce the likelihood of accidental use (like we do
for the 'g' register). The same applies for X3 (GP) when the -shared flag is given.

Updates #47100

Change-Id: I72e82b5ca3f80c46a781781345ca0432a4111b74
Reviewed-on: https://go-review.googlesource.com/c/go/+/351859
Trust: Joel Sing <joel@sing.id.au>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>