[gcc r14-8344] LoongArch: doc: Add attribute descriptions defined in the target-supports.exp.

[LuluCheng]

https://gcc.gnu.org/g:e861d76076e213d036f880b722a885918ee8cc58

commit r14-8344-ge861d76076e213d036f880b722a885918ee8cc58
Author: chenxiaolong <chenxiaolong@loongson.cn>
Date:   Wed Jan 17 09:24:06 2024 +0800

    LoongArch: doc: Add attribute descriptions defined in the target-supports.exp.
    
    gcc/ChangeLog:
    
            * doc/sourcebuild.texi: Add attributes for keywords.

Diff:
---
 gcc/doc/sourcebuild.texi | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/gcc/doc/sourcebuild.texi b/gcc/doc/sourcebuild.texi
index 8082100a3c9..6c33237ac78 100644
--- a/gcc/doc/sourcebuild.texi
+++ b/gcc/doc/sourcebuild.texi
@@ -2352,6 +2352,26 @@ AArch64 target that is able to generate and execute armv8.3-a FJCVTZS
 instruction.
 @end table
 
+@subsubsection LoongArch specific attributes
+
+@table @code
+@item loongarch_sx
+LoongArch target that generates instructions for SX.
+
+@item loongarch_asx
+LoongArch target that generates instructions for ASX.
+
+@item loongarch_sx_hw
+LoongArch target that is able to generate and execute SX code.
+
+@item loongarch_asx_hw
+LoongArch target that is able to generate and execute ASX code.
+
+@item loongarch_call36_support
+LoongArch binutils supports call36 relocation.
+
+@end table
+
 @subsubsection MIPS-specific attributes
 
 @table @code