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ARROW-7404: [C++][Gandiva] Fix utf8 char length error on Arm64

Current code checks if a UTF-8 eight-bit code unit is within 0x00~0x7F
by "if (c >= 0)", where c is defined as "char". This checking assumes
char is always signed, which is not true[1]. On Arm64, char is unsigned
by default and causes some Gandiva unit tests fail.

Fix it by casting to "signed char" explicitly.

[1] Cited from https://en.cppreference.com/w/cpp/language/types
The signedness of char depends on the compiler and the target platform:
the defaults for ARM and PowerPC are typically unsigned, the defaults
for x86 and x64 are typically signed.

Signed-off-by: Yibo Cai <yibo.cai@arm.com>

Closes #6043 from cyb70289/utf8_char_len and squashes the following commits:

a18f43d0d <Yibo Cai> ARROW-7404:  Fix utf8 char length error on Arm64

Authored-by: Yibo Cai <yibo.cai@arm.com>
Signed-off-by: Pindikura Ravindra <ravindra@dremio.com>