gdb: LoongArch: Change default char data type to signed

According to "Procedure Call Standard for the LoongArch Architecture" [1],
for all base ABI types of LoongArch, the char data type in C is signed by
default, so change the char data type to signed in gdb/loongarch-tdep.c.

Before this patch:

make check-gdb TESTS="gdb.base/sizeof.exp"

=== gdb Summary ===

 # of expected passes		75
 # of unexpected failures	1

make check-gdb TESTS="gdb.base/charset.exp"

=== gdb Summary ===

 # of expected passes		277
 # of unexpected failures	6
 # of unsupported tests		1

After this patch:

make check-gdb TESTS="gdb.base/sizeof.exp"

=== gdb Summary ===

 # of expected passes		76

make check-gdb TESTS="gdb.base/charset.exp"

=== gdb Summary ===

 # of expected passes		283
 # of unsupported tests		1

[1] https://github.com/loongson/la-abi-specs/blob/release/lapcs.adoc#appendix-c-data-types-and-machine-data-types

Signed-off-by: Hui Li <lihui@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>