debug/elf: add R_RISCV_32_PCREL relocation

This were missed in CL 107339 as it is not documented (yet) in
https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md

But binutils already uses it. See
https://github.com/riscv/riscv-elf-psabi-doc/issues/36

Change-Id: I1b084cbf70eb6ac966136bed1bb654883a97b6a9
Reviewed-on: https://go-review.googlesource.com/134015
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/debug/elf/elf.go

@@ -2424,6 +2424,7 @@ const (
 	R_RISCV_SET8          R_RISCV = 54 /* Local label subtraction */
 	R_RISCV_SET16         R_RISCV = 55 /* Local label subtraction */
 	R_RISCV_SET32         R_RISCV = 56 /* Local label subtraction */
+	R_RISCV_32_PCREL      R_RISCV = 57 /* 32-bit PC relative */
 )
 
 var rriscvStrings = []intName{
@@ -2480,6 +2481,7 @@ var rriscvStrings = []intName{
 	{54, "R_RISCV_SET8"},
 	{55, "R_RISCV_SET16"},
 	{56, "R_RISCV_SET32"},
+	{57, "R_RISCV_32_PCREL"},
 }
 
 func (i R_RISCV) String() string   { return stringName(uint32(i), rriscvStrings, false) }