Drop 'missing mapping symbols' warning for arm and aarch64.

Drop 'missing mapping symbols' warning for arm and aarch64.

The psABI (AAELF) says this about mapping symbols:

    4.6.5.1 Section-relative mapping symbols

    Mapping symbols defined in a section define a sequence of
    half-open address intervals that cover the address range of the
    section. Each interval starts at the address defined by the
    mapping symbol, and continues up to, but not including, the
    address defined by the next (in address order) mapping symbol or
    the end of the section. A section must have a mapping symbol
    defined at the beginning of the section; however, if the section
    contains only data then the mapping symbol may be omitted.

That makes it pretty clear that, in the absence of a mapping symbol,
the entire section can be treated as data.

gold/ChangeLog:
	* aarch64.cc (AArch64_relobj::scan_errata): Drop missing symbol warning.
	* arm.cc (Arm_relobj::scan_section_for_cortex_a8_erratum): Drop missing
	symbol warning.