[XCOFF] Classify XMC_RO symbols as data in getSymbolType()

`XCOFFObjectFile::getSymbolType()` currently returns `ST_Other` for symbols with storage mapping class `XMC_RO` (read-only data in the TEXT segment). These should be classified as `ST_Data` so that symbolizers and other consumers correctly identify them as data symbols rather than falling through to the unknown case.
`XMC_RO` symbols appear in real AIX binaries and are expected to be data symbols. The `isFunction()` check already correctly excludes them from function classification (only XMC_PR and XMC_GL are functions), but the remainder of `getSymbolType()` needs to check the storage mapping class and return `ST_Data` for `XMC_RO`.