String endianness is incorrect: big-endian strings cannot be read on little-endian machines

According to the specification:
> The character set is Unicode in the UTF-8 encoding scheme. The UTF-8 octets (8-bit bytes) are packed four per word, following the little-endian convention (i.e., the first octet is in the lowest-order 8 bits of the word).

This means that bytes must be swapped before reading a big-endian spv file on a little-endian machine, and vice versa. For example, `glslang` outputs big-endian encoded spv binary files on big-endian machines. But when disassembling this file via `spirv-dis` on little-endian machines, only words and operands are handled properly via [spvFixWord](https://github.com/KhronosGroup/SPIRV-Tools/blob/c3a9ffd74f8b27434d220f2e48a98ac6cd72d178/source/spirv_endian.cpp#L34-L42), strings are handled in host endianness, which is not correct.

More specifically:
"GLSL.std.450" in big-endian encoding (or on big-endian machines), the first octet 'G' should be in the lowest-order byte, which is the fourth byte in a word. Then in the file (or memory), from the first byte to the last byte, from left to right is, 'L''S''L''G' 'd''t''s''.' '0''5''4''.' '\0''\0''\0''\0', 16 bytes and 4 words in total.
When reading this big-endian encoded file:
On big-endian machines, reinterpret the each consecutive 4 bytes as unit32_t, and use bit shift to obtain the first octet, like `(word >> 0) & 0xFF`. We will get the fourth byte, which is 'G', and this is correct.
On little-endian machines, the result of the bit operation is the first byte, which is 'L'. This is not correct, because there is no call to `spvFixWord`.

I'm making a compiler in Java myself and can selectively output spv binary files in little-endian or big-endian (the default is host endianness). I encountered this issue when running `spirv-dis`:
```
; SPIR-V
; Version: 1.5
; Generator: Khronos; 0
; Bound: 25
; Schema: 0
OpCapability Shader
error: 2: Invalid extended instruction import 'LSLGdts.054.'
```

A related issue is #149 and PR #4622, but it does *not* fix this issue.

My spirv-dis version: `SPIRV-Tools v2023.6 v2023.6.rc1-50-gdc667644`
Here is my spv binary file for testing purposes, git describes this file as `Khronos SPIR-V binary, big-endian, version 0x010500, generator 00000000`, my CPU is little-endian
[test_shader.zip](https://github.com/KhronosGroup/SPIRV-Tools/files/14401424/test_shader.zip)