arm64: fall back to ramdump ELF prstatus notes when note_buf_t missing

[miniLQ]

When note_buf_t does not exist in kernel's debug info (e.g. stripped vmlinux), SIZE(note_buf) returns -1 causing a fatal 'invalid structure size' error. Add a check at the beginning of arm64_get_crash_notes() to detect this condition, print a WARNING, and fall back to reading NT_PRSTATUS notes directly from the ramdump's ELF headers.

[miniLQ]

crash-android 9.0.2++
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NOTE: setting vabits_actual to: 39

NOTE: setting phys_offset to: 0x80000000

NOTE: setting kimage_voffset to: 0xffffffbf88000000

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crash-android: invalid structure size: note_buf
FILE: arm64.c LINE: 5103 FUNCTION: arm64_get_crash_notes()

[./crash-android] error trace: 5630e5db12d3 => 5630e5e74869 => 5630e5e98e2b => 5630e5e98d9d