[PATCH] RISC-V: fix build issue with gcc 4.9.x

[Romain Naour <romain.naour@gmail.com>]

GCC should still build with GCC 4.8.3 or newer [1]
using C++03 by default. But a recent change in
RISC-V port introduced a C++11 feature "std::log2" [2].

Use log2 from the C header, without the namespace [3].

[1] https://gcc.gnu.org/install/prerequisites.html
[2] https://gcc.gnu.org/git/?p=gcc.git;a=patch;h=7caa1ae5e451e780fbc4746a54e3f19d4f4304dc
[3] https://stackoverflow.com/questions/26733413/error-log2-is-not-a-member-of-std

Fixes:
https://gitlab.com/buildroot.org/toolchains-builder/-/jobs/4202276589

gcc/ChangeLog:
	* config/riscv/genrvv-type-indexer.cc: Use log2 from the C header, without
	the namespace.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
---
 gcc/config/riscv/genrvv-type-indexer.cc | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/gcc/config/riscv/genrvv-type-indexer.cc b/gcc/config/riscv/genrvv-type-indexer.cc
index e677b55290c..eebe382d1c3 100644
--- a/gcc/config/riscv/genrvv-type-indexer.cc
+++ b/gcc/config/riscv/genrvv-type-indexer.cc
@@ -115,9 +115,9 @@ same_ratio_eew_type (unsigned sew, int lmul_log2, unsigned eew, bool unsigned_p,
   if (sew == eew)
     elmul_log2 = lmul_log2;
   else if (sew > eew)
-    elmul_log2 = lmul_log2 - std::log2 (sew / eew);
+    elmul_log2 = lmul_log2 - log2 (sew / eew);
   else /* sew < eew */
-    elmul_log2 = lmul_log2 + std::log2 (eew / sew);
+    elmul_log2 = lmul_log2 + log2 (eew / sew);
 
   if (float_p)
     return floattype (eew, elmul_log2);
-- 
2.34.3