Fangrui Song 6ab8927931 [RISCV] Support clang -fpatchable-function-entry && GNU function attribute 'patchable_function_entry' ...

Similar to D72215 (AArch64) and D72220 (x86).

```
% clang -target riscv32 -march=rv64g -c -fpatchable-function-entry=2 a.c && llvm-objdump -dr a.o
...
0000000000000000 <main>:
       0: 13 00 00 00   nop
       4: 13 00 00 00   nop

% clang -target riscv32 -march=rv64gc -c -fpatchable-function-entry=2 a.c && llvm-objdump -dr a.o
...
00000002 <main>:
       2: 01 00         nop
       4: 01 00         nop
```

Recently the mainline kernel started to use -fpatchable-function-entry=8 for riscv (https://git.kernel.org/linus/afc76b8b80112189b6f11e67e19cf58301944814).

Differential Revision: https://reviews.llvm.org/D98610

2021-03-16 10:02:35 -07:00

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